In manufacture of wafers, such as silicon wafers, conventionally, an ingot is sliced into wafers, the wafers are subjected to, in sequence, lapping, etching, and other processes, and at least a main surface of the wafers is then mirror-polished. This polishing process employs a single-side polishing apparatus for polishing one surface of wafers and a double-side polishing apparatus for simultaneously polishing both surfaces of wafers.
A double-side polishing apparatus includes an upper turn table and a lower turn table, and polishing pads, which are typically made of a nonwoven fabric, are attached to the respective turn tables. FIG. 5 illustrates a four-ways type of a double-side polishing apparatus that has a planetary gear structure in which a sun gear 107 is disposed at the center and an internal gear 108 is disposed at the outer circumference. With such a double-side polishing apparatus, wafers are inserted into the respective wafer holding holes 103 formed in a carrier 102 and held therein, a polishing agent is supplied to the wafers from above the wafers, front and back surfaces of the wafers are pressed by the respective polishing pads while the upper and lower turn tables are rotated and the carrier 102 is rotated about its axis and revolved between the sun gear 107 and the internal gear 108. The both surfaces of the wafers are thereby simultaneously polished.
There is a problem in that while a polishing batch is repeated with a double-side polishing apparatus without replacing a polishing pad with a new one, variation in wafer shape among polished wafers gradually increases. This problem is attributed mainly to the lifetime of the polishing pad. As duration of use of the polishing pad increases, excessive polishing at the outer circumference of wafers, which is referred to as an outer circumference sag, easily occurs due to influence of compressibility variation of and clogging of the polishing pad. It is accordingly necessary to dress the surface of the polishing pad in order to eliminate this problem.
A polishing pad for use in a double-side polishing apparatus is typically dressed as follows: Dressing plates are inserted into the respective holding holes of a normal carrier used in polishing of wafers or a carrier dedicated to dressing; this carrier is interposed between the upper and lower turn tables; and the apparatus is operated in the same way as normal polishing.
It is important to dress a polishing pad such that the surface of the polishing pad is carded to stabilize and improve polishing capability. A dressing plate having diamond pellets etc., attached to both its surfaces may be used to achieve such dressing.
When double-side polishing of wafers, such as silicon wafers, is repeated in a batch manner, dressing for the carding of a polishing pad is performed to ensure polishing stability at the beginning of use (the starting up) of a new one and at intervals of prescribed batches. As a polishing batch is repeated, a polishing pad surface roughened by such dressing is gradually smoothed and the outer circumference sag of wafers thereby occurs more frequently, so the dressing performed at the intervals is appropriately needed.
The direction (a dressing direction) of a dressing condition of a polishing pad may be represented by a grain direction and a direction opposite the grain direction, and is determined depending on the direction in which a dress plate is revolved during dressing. Patent Document 1 discloses a method of polishing a wafer with a carrier revolved in a direction opposite the direction in which a dress plate is revolved during dressing in order to obtain a highly flat wafer.